1. Field of the Invention
This invention relates to a method for increasing the amount of cenospheres in a fly ash sample.
2. Description of the Related Art
Coal fired boilers are widely used to generate steam for producing electricity. A common form of boiler uses a pulverized coal that is injected into a furnace. The process of burning coal in a boiler produces fly ash. Some of the recovered fly ash is commercially usable in concrete, concrete products, cement production, sewage sludge stabilization, pavement base materials, lightweight aggregate, reinforced plastics, and other miscellaneous purposes. The remaining fly ash must generally be disposed of by landfilling since it has no commercial value. It is well known that landfill space is rapidly dwindling in many regions and that the construction of new landfills is very costly. Therefore, much effort has been directed at finding uses for fly ash so that the fly ash does not need to be landfilled.
The composition of fly ash can vary depending on the composition of coal that generated the fly ash. Therefore, material specifications have been developed for fly ash that is to be used in specific applications. For example, fly ash that is used as a filler in concrete should meet the specifications described in American Society for Testing and Materials, “Standard Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete”, Designation: C 618. This ASTM specification indicates that fly ash particles are made largely of silicon dioxide, aluminum oxide and iron oxide. The particles in fly ash have different types of structures. Some particles in fly ash are solid. Other particles in fly ash are hollow and are called cenospheres. A cenosphere is a lightweight, inert, hollow sphere filled with inert air or gas. Cenospheres are hard and rigid, light, waterproof and insulative. Due to the hollow structure, cenospheres have lower density (e.g., some cenospheres have a density below 1 g/cc) as compared to solid fly ash particles. Cenospheres have traditionally been reported to represent about 1-3% by weight of the total fly ash produced.
In the composite materials field, there is increasing interest in ultra light energy-absorbing materials and structures which incorporate hollow particles. Accordingly, cenospheres have been added to polymers and there is growing interest in incorporating them in metals. Fine and sized cenospheres can be sold at attractive prices to this segment of industry providing syntactic foams and cellular solids. In addition, there is a growing market for fine solid microspheres to produce reinforced polymers, metals and ceramics.
Methods are known for recovering cenospheres from fly ash. In one method, cenospheres lighter than water are skimmed off of ash ponds (as many cenospheres float due to their lower density), dried and sold for use. U.S. Pat. No. 4,652,433 describes another method in which fly ash is screened into a coarse fraction and a fine fraction, and cenospheres are removed from the coarse fraction by mixing the coarse fraction with water such that the cenospheres float on the water and are removed by skimming. However, these processes do have disadvantages. For example, these methods only collect cenospheres with a density of less than 1.0 g/cc as only cenospheres of these densities float on water. Also, the fly ash that is produced from burning subbituminous western coal includes significant amounts of calcium compounds. For example, this fly ash may include 10% or more lime. These high calcium fly ashes (such as ASTM C 618 Class C fly ash) have cementitious properties and therefore, when mixed with water these fly ashes rapidly harden and the remainder cannot then be used for other purposes such as a cementitious material in the production of concrete. In the case of Class F fly ash, a dry method also provides the advantage of not expending energy to dry the remainder after separation of cenospheres for other uses. Another disadvantage is that, many cenospheres are entrapped in agglomerated and/or hardened masses before flotation occurs. Furthermore, the known methods for recovering cenospheres from fly ash do not allow for recovery of cenospheres of controlled sizes and densities. As a result, the properties of polymeric composites including cenospheres have not been optimized due to lack of availability of cenospheres of controlled sizes and densities.
Thus, there is a need for a method for recovering cenospheres from fly ash that has cementitious properties. There is also a need for a method for recovering cenospheres of narrow size ranges and densities from fly ash.